1. Field of the Invention
This invention relates to a misfire-detecting system for an internal combustion engine, and more particularly to a misfire-detecting system adapted to detect a misfire attributable to the fuel supply system.
2. Prior Art
In an internal combustion engine having spark plugs, a misfire can occur, in which normal ignition does not take place at one or more of the spark plugs. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, for example, due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel or unburnt fuel to the spark plug, or abnormality in the sparking voltage supply system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage detecting means which detects sparking voltage, i.e. voltage across electrodes of the spark plug, and misfire-determining means which determines that a misfire has occurred based on a detected value of the sparking voltage, e.g. when a time period over which the detected value of the sparking voltage exceeds a predetermined reference value (U.S. Pat. No. 5,215,067).
FIG. 1a shows a timing chart for explaining a method of misfire determination carried out by the proposed system, in which are shown changes in the sparking voltage V and a comparative level (the predetermined reference value) VCOMP occurring in an example of a misfire determination. As shown in FIG. 1a, when the sparking voltage V exceeds the comparative level VCOMP which is set depending on the sparking voltage, a comparative signal pulse P1 is at a high level. If this state in which the comparative signal pulse P1 is at the high level continues over a predetermined time period, it is determined that a misfire has occurred. Therefore, in the example shown in FIG. 1a, it is determined that a misfire occurred in the first sparking operation at a time point t1, and that normal firing has occurred in the following sparking operation at a time point t2.
However, if the above method of misfire determination is applied to an engine having a large number of cylinders or an engine adapted to operate at a very high engine speed, there arises the following problems:
Referring to FIG. 1b, in this example of misfire determination as well, at the first sparking operation (at a time point t1), it is determined that a misfire has occurred since a comparative pulse P2 has continued to be at a high level over a predetermined time period. However, in the case of the engine having a large number of cylinders or the engine operating at a very high engine speed, a repetition period of ignition becomes so short that sparking discharge can be performed for the following cylinder at a time point t2 before sparking voltage detected or charged for the present sparking operation has attenuated. In such a case, as indicated by Q1 in FIG. 1b, the sparking voltage V detected is increased or pushed up. Therefore, a duration of the high level of the comparative pulse P2 occurring for this sparking operation become longer than a corresponding one shown in FIG. 1a, which can lead to an erroneous determination of a misfire in spite of the fact that firing has been normally performed. In this respect, the proposed system has room for improvement.